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WB-57
WB57
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Polarimetric Scanning Radiometer - Original Scanhead

The PSR/A scanhead provides either full-Stokes vector or tri-polarimetric sensitivity at the radiometric bands of 10.7, 18.7, and 37 GHz, and thus is well suited for the NPOESS Integrated Program Office’s internal government (IG) studies of ocean surface wind vector measurements. PSR data has been used to demonstrate the first-ever retrieval of ocean surface wind fields using conically-scanned polarimetric radiometer data. The results have suggested that the NPOESS specification for wind vector accuracy will be achievable with a polarimetric two-look system.

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Polarimetric Scanning Radiometer

The Polarimetric Scanning Radiometer (PSR) is a versatile airborne microwave imaging radiometer developed by the Georgia Institute of Technology and the NOAA Environmental Technology Laboratory (now NOAA/ESRL PSD) for the purpose of obtaining polarimetric microwave emission imagery of the Earth's oceans, land, ice, clouds, and precipitation. The PSR is the first airborne scanned polarimetric imaging radiometer suitable for post-launch satellite calibration and validation of a variety of future spaceborne passive microwave sensors. The capabilities of the PSR for airborne simulation are continuously being expanded through the development of new mission-specific scanheads to provide airborne post-launch simulation of a variety of existing and future U.S. sensors, including CMIS, ATMS, AMSU, SSMIS, WindSat, TMI, RAMEX, and GEM.

The basic concept of the PSR is a set of polarimetrc radiometers housed within a gimbal-mounted scanhead drum. The scanhead drum is rotatable by the gimbal positioner so that the radiometers (Figure 2.) can view any angle within ~70° elevation of nadir at any azimuthal angle (a total of 1.32 pi sr solid angle), as well as external hot and ambient calibration targets. The configuration thus supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. The PSR was designed to provide several specific and unique observational capabilities from various aircraft platforms. The original design was based upon several observational objectives:

1. To provide fully polarimetric (four Stokes' parameters: Tv, Th, TU, and TV) imagery of upwelling thermal emissions at several of the most important microwave sensing frequencies (10.7, 18.7, 37.0, and 89.0 GHz), thus providing measurements from X to W band;
2. To provide the above measurements with absolute accuracy for all four Stokes' parameters of better than 1 K for Tv and Th, and 0.1 K for TU and TV;
3. To provide radiometric imaging with both fore and aft look capability (rather than single swath observations);
4. To provide conical, cross-track, along-track, and spotlight mode scanning capabilities; and
5. To provide imaging resolutions appropriate for high resolution studies of precipitating and non-precipitating clouds, mesoscale ocean surface features, and satellite calibration/validation at Nyquist spatial sampling.

The original system has been extended - as discussed below - to greatly exceed the original design objectives by providing additional radiometric channels and expanded platform capabilities.

The PSR scanhead was designed for in-flight operation without the need for a radome (i.e., in direct contact with the aircraft slipstream), thus allowing precise calibration and imaging with no superimposed radome emission signatures. Moreover, the conical scan mode allows the entire modified Stokes' vector to be observed without polarization mixing.

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Applanix POS System

POS AV is a hardware and software system specifically designed for direct georeferencing of airborne sensor data. By integrating precision GNSS with inertial technology, POS AV enables geospatial projects to be completed more efficiently, effectively, and economically. POS AV is engineered for aerial cameras, scanning lasers, imaging sensors, synthetic aperture radar, and LIDAR technology.

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NOAA Pressure and Temperature

In order to make an accurate temperature and pressure measurement, a Weston digital pressure temperature transducer is used to measure both static and ram pressure. These transducers are accurate to within +/- 0.01 % of full-scale or +/- 0.1 mbar. When the aircraft was manufactured, two ports on either side of the aircraft were placed at positions where the air moving across the skin is perpendicular to the port. These ports are connected together and to the static pressure transducer. The ram pressure measurement consists of a forward-looking tube with a wideangle opening connected to the ram pressure transducer. The ram pressure is calculated by subtracting the static pressure from this measurement.

The temperature probes consist of a slow and fast responding type 102 probe from Goodyear Aerospace Corporation. The platinum wire temperature sensor in the type 102 probe is calibrated to less than +/- 0.1 degree.

Data is gathered once every second from these probes using a custom data system. The Weston pressure transducers are held at a constant temperature of 50 degrees Celsius in order to reduce temperature effects on the measurement and in order to prevent condensation within the sensor. The analog to digital converters are also held at a relatively constant temperature, and a thousand samples from each channel is averaged each second. This over sampling results in a precision of 0.03 degrees in temperature and 0.03 mbars in pressure. We estimate the total accuracy of these measurements in flight to be +/- 0.5 degrees for temperature and +/- 0.5 mb for pressure.

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Nuclei-Mode Aerosol Size Spectrometer

The nucleation-mode aerosol size spectrometer (NMASS) measures the concentration of particles as a function of diameter from approximately 4 to 60 nm. A sample flow is continuously extracted from the free stream using a decelerating inlet and is transported to the NMASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Fig. 1. Each CNC is tuned to measure the cumulative concentration of particles larger than certain diameter. The minimum detectable diameters for the 5 CNCs are 4.0, 7.5, 15, 30 and 55 nm, respectively. An inversion algorithm is applied to recover a continuous size distribution in the 4 to 60 nm diameter range.

The NMASS has been proven particularly useful in measurements of nucleation-mode size distribution in environments where concentrations are relatively high and fast instrumental response is required. The instrument has made valuable measurements vicinity of cirrus clouds in the upper troposphere and lower stratosphere (WAM), in the near-field exhaust of flying aircraft (SULFUR 6), in newly created rocket plumes (ACCENT), and in the plumes of coal-fired power plants (SOS ’99). The instrument has flown on 3 different aircraft and operated effectively at altitudes from 50 m to 19 km and ambient temperatures from 35 to -80ºC.

Accuracy. The instrument is calibrated using condensationally generated particles that are singly charged and classified by differential electrical mobility. Absolute counting efficiencies are determined by comparison with an electrometer. Monte carlo simulations of the propagation of uncertainties through the numerical inversion algorithm and comparison with established laboratory techniques are used to establish accuracies for particular size distributions, and may vary for different particle size distributions. A study of uncertainties in aircraft plume measurements demonstrated a combined uncertainty (accuracy and precision) of 38%, 36% and 38% for number, surface and volume, respectively.

Precision. The precision is controlled by particle counting statistics for each channel. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are recorded with 10 Hz resolution, and the instrument has demonstrated response times of this speed in airborne sampling. However the effective response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements with high concentrations of nucleation-mode particles may be processed at 10 Hz.

Specifications: Weight is approximately 96 lbs, including an external pump. External dimensions are approximately 15”x16”x32”. Power consumption is 350 W at 28 VDC, including the pump.

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Non-dispersed Infrared Airborne CO2 Detector

NIRAD consists of three systems: (1) CO2 detector, (2) power and data acquisition, and (3) gas-handling. All three systems have flown previously. The CO2 detector was first flown in 1999 as part of CORE+ instrument during RISO and ACCENT and again in 2004 during PUMA-A. There have been no changes to the detector, other than inspection and routine maintenance. The power and data acquisition system were new for PUMA-A, and are flown here without change, other than to software. The gas-handling system is the same as that flown in May 2004, except that it is now packaged into a single box that contains the detector and power/data system.

The detector is packaged in a vacuum housing to facilitate management of temperature and pressure. At power-up the housing is pumped down to ~300 hPa by one stage of a diaphragm pump and held at this pressure throughout the flight. Thus, at pressure altitudes < 300 hPa the pressure within the housing is above ambient. By design, if the pressure differential is significantly greater than about 5 psi, the O-ring seals leak. A redundant additional mechanical safety relief valve (set for ~15 psi or less) is placed on the housing.

Two 1.2 L epoxy-coated, fiber-wrapped aluminum bottles (DOT rated and certified) are filled to ~1600 psi before flight with zero air doped with CO2. These ‘standards’ are sampled repeatedly during flight to provide an accurate standard for reference to the NOAA/CMDL CO2 scale. Two-stage regulators provide a service pressure of ~25-30 psig throughout flight. The bottles and regulators are backed with safety relief valves.

The diaphragm pump is current-limited for a ‘soft start’ (that is, there is no electrical surge on startup, allowing for use of compact, highly efficient Vicor VI-100 DC/DC converters.

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NPOESS Airborne Sounder Testbed - Microwave

The NAST-M currently consists of two radiometers covering the 50-57 GHz band and a set of spectral emission measurements within 4 GHz of the 118.75 GHz oxygen line with eight single sideband and 9 double sideband channels, respectively. To be added prior to CRYSTAL-FACE are five double side band channels within 4 GHz of the 183 GHz water vapor line and a single band channel at 425 GHz. For clear air, the temperature and water vapor information provided by the 50-57 GHz, 118 GHz, and 183 GHz channels is largely redundant; but, for cloudy sky conditions the three bands provide information on the effects of precipitating clouds on the temperature and water vapor profile retrievals and enables sounding through the non-precipitating portion of the cloud, a feature particularly important for CRYSTAL-FACE.

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HyMap

The HyMap scanner, built by Integrated Spectronics Inc of Sydney, Australia, has four spectrometers in the interval 0.45 to 2.45 micrometers excluding the two major atmospheric water absorption windows. The bandwidths are not constant, but vary between 15 and 18 nanometers. The scanner also has an on-board bright source calibration system, which is used to monitor the stability of the signal. The signal/noise ratio measured outside the aircraft with a sun angle of 30° and a 50% reflectance standard is more than 500/1 except near the major atmospheric water absorption bands. The scanner is mounted on a hydraulically actuated Zeiss-Jena SM 2000 stabilized platform. The platform provides +/- 5 degrees of pitch and roll correction. The yaw can be offset by +/- 20 degrees with +/- 8 degrees of stabilization. The platform provides a residual error in nadir pointing of less than 1 degree and reduces aircraft motion effects by a factor ranging from 10:1 to 30:1.

The basic HyMap specifications are:

IFOV: 2.5 mr along track, 2.0 mr across track (Spatial resolution 3.5–10 m)
FOV: 62 degrees (512 pixels)

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High-Sensitivity Fast-Response CO2 Analyzer

The high-sensitivity fast response CO2 instrument measures CO2 concentrations in situ using the light source, gas cells, and solid-state detector from a modified nondispersive infrared CO2 analyzer (Li-Cor, Inc., Lincoln, NE). These components are stabilized along the detection axis, vibrationally isolated, and housed in a temperature-controlled pressure vessel. Sample air enters a rear-facing inlet, is preconditioned using a Nafion drier (to remove water vapor), then is compressed by a Teflon diaphragm pump. A second water trap, using dry ice, reduces the sample air dewpoint to less than 70C prior to detection. The CO2 mixing ratio of air flowing through the sample gas cell is determined by measuring absorption at 4.26 microns relative to a reference gas of known concentration. In-flight calibrations are performed by replacing the air sample with reference gas every 10 minutes, with a low-span and a high-span gas every 20 minutes, and with a long-term primary standard every 2 hours. The long-term standard is used sparingly and serves as a check of the flight-to-flight accuracy and precision of the measurements, augmented by ground-based calibrations before and after flights.

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Meteorological Measurement System

The Meteorological Measurement System (MMS) is a state-of-the-art instrument for measuring accurate, high resolution in situ airborne state parameters (pressure, temperature, turbulence index, and the 3-dimensional wind vector). These key measurements enable our understanding of atmospheric dynamics, chemistry and microphysical processes. The MMS is used to investigate atmospheric mesoscale (gravity and mountain lee waves) and microscale (turbulence) phenomena. An accurate characterization of the turbulence phenomenon is important for the understanding of dynamic processes in the atmosphere, such as the behavior of buoyant plumes within cirrus clouds, diffusions of chemical species within wake vortices generated by jet aircraft, and microphysical processes in breaking gravity waves. Accurate temperature and pressure data are needed to evaluate chemical reaction rates as well as to determine accurate mixing ratios. Accurate wind field data establish a detailed relationship with the various constituents and the measured wind also verifies numerical models used to evaluate air mass origin. Since the MMS provides quality information on atmospheric state variables, MMS data have been extensively used by many investigators to process and interpret the in situ experiments aboard the same aircraft.

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